when you have a default value, it over rides the empty string.
by assigning a default value in the data base and not assigning one in the upload behavior, the database default over rides the empty string.
if you set a default in the Database and a default in the upload behavior, the upload default would be used.
having to add a form element to your page is a far cry from having to buy something.
Yes, I agree it is an extra step. I also agree it is one that is not intuitive. but that is the work flow for using file upload on an update page.